Electrical resistance alloys



cost electrical resistance temperatures are sistance properties of Patented Oct. 30, 1945 l I assmso snac'rmcan RESISTANCE annoys Hugh 5. Cooper,'Cleveland, Ohio No Drawing. Application February 17, 1945, Serial No. 578,539

Claims.

'Ihis invention relates to electrical resistance alloys and to heating elements comprised thereof. one object of the invention is to provide a relatively low cost electrical resistance alloy.

Another object is to provide a, relatively low heating element. A further object is to provide'an improved electrical resistance heating element.

Other objects will be apparent as the invention is more fully hereinafter disclosed.

This application is a continuation-in-part application of application Serial No. 505,038nled October 5, 1943, entitled Rust and oxidation re sistant alloys.

In accordance discovered that an iron num, chromium and columbium, each within relatively narrow and critical percentage ranges, is characterized by being highly ductileand cold workable to relatively small diameter wire or thin sectioned strip form, oxidation and corrosion at atmospheric temperatures, highly resistant to oxidation at elevated temperatures up to about l000 to 1100 C. and, in addition. is characterized by sistance approximating 600 ohms per circular mil foot at 25' C. and by a temperature coemcient of electrical resistivity per degree C. approximating .0005'to .0008 between 50 and 650 C.

These physical and electrical characteristics of the alloy composition of the present invention adapt the material to wide utility as an electrical resistance heating element in various types of electrical devices. I

In .the alloy composition of the present invention, I have discovered that the electrical properties and the resistance to oxidation at elevated imparted to the alloy by an with the above objects I have aluminum content and a chromium content within the range 2 to 4% and that resistance to oxidation and corrosion at atmospheric temperatures is imparted by columbium within the range .25 to 2%. In my alloy. carbon is preferably substantially excluded and in no event must exceed 20% and when carbon is present in amounts up to 20% the columbium content is increased from the minimumof 30% to an amount at least 25% in excess of 8 times the carbon. to prevent the formation of chromium carbide compounds detrimentally affecting the chromium content of the alloy thereby changing or altering, the desired electrical properties and high temperature oxidation rethe alloy. Manganese and silicon in amounts up to 50% are permissible con an electrical re-- within the range 5.25 to 7.5%

alloy containing alumialloy known in high resistance to surface' 59 hours was obtained I 0.25% aluminum life at 1050' stituents of the alloy but are not essential thereto except insofar as they may be utilized as deoxidizing agents in the iron base of the alloy in the manufacture of the alloy, in accordance with best prior art practice.

As specific examples of the present invention the following examples are given;

ass

proximates .65%; manganese 50%; silicon-20%;

and the carbon .04% withthe balance iron. Under similar test conditions the nickel-chromium the art under the trade-name "Nichrome" evidences a life of about hours. 0f the various alloy compositions possible under the narrow range of constituents above given the alloy containing about 6.5% A1 and 3.25% Cr or Al and Cr in thevratio of 2:1 with about .05

' carbon and .65% columbium, appears to provide the longest life under the accelerated life test conditions customarily applied to such electrical resistance alloys. This alloy in a plurality of tests has consistently shown a total life at 980 C.'of over 1200 hours to a 5% change in resistance without burn-out. The electrical resistance of this specific alloy approximates 575 ohms per circular mil foot and the material consistently shows a maximum temperature coefficient per degree C.

between 50 C. and 650 C. of .000543.

' The alloy of this specific composition is highly ductile and may be cold drawn into small diameter wire or cold-rolled into thin sectioned strip material and may be readily coiled about a mandrel into relatively sm i1 diameter heater element coils.

In the alloy composition of the present invention any increase in the aluminum or chromium content above the maximum limits of 7.0% and 4%, respectively, or any decrease of the same constituents below the minimum limits of 5.20% and 2.0%, respectively, markedly eflects the oxidation resistance of the alloy at hish temperatures. As an indication of this. in an alloy containinfl about 8.66% aluminum and about 2.5% chromium. an operating life at 1050 on test; in an alloy about and about 1.25% chromium a C. of only 89 hours was obtained on test. In the first alloy the Al content is above imum percentage the maximum percentage specified and the chromium is well within the range specified, yet the test life at 1050 0.18 about one-third that oi nickel-chromium (ni-chrome) In the second alonly about one-half that of nickel-chromium (nichrome).

Further experiments-have shown that an increase in the chromium content or the first alloy noted immediately above, to about 5% improving the test life, actually lowered the test life from 59 hours to 39 hours. To demonstrate the converse of the above, an alloy containing 4.65% Al and 6.89% Cr showed a test life of only 22 hours.

Various other tests haveindicated that aluminum over 7.5% and Cr under 2.0% are each det-g is approximately one-half of the aluminum content an operating life from 2 to 3 times as that (hi-chrome) of Al and Cr specified the ratio instead oiv and the chromium content.

2 to 1 appears critical to a long operating life at high temperatures.

Having herelnabove disclosed the present invention broadly and specifically and having given several specific examples oi the same, it is bea lieved apparent that the same may be varied widely without essential departure therefrom and all such modifications and departures are con templated as may fall within the scope-or the following claims:

What I cl is:

1. An electrical resistance alloy, sisting 01 from 5.25 %-7.50% Al, Cb, balance Fe.

2. An electrical resistance alloy,.said alloy consisting of from 5.25-7.50% Al, 2-4% Cr, 25-20% Cb, carbon not over 20% with the Cb content at least .25% in excess of 8 times the carbon, balance Fe.

3. An electrical resistance alloy, sisting of 5.25-7.50% Al, 24% Cr; notpover each of Mn and-Si, carbon not over .20%, Cb .25% in excess of 8 times the carbon, balance Fe.

4. An electrical resistance alloy, said alloy consisting of A1 5.25-7.50%, Cr approximately onesaid alloy consaid alloy con- 24% Cr, .25-2% half the Al, carbon not over .20%, Ch 25% in excess of '8 times the carbon, balance Fe. 7

5. An electrical resistance alloy, said alloy consisting of A1 5.25-'7.50%, Cr approximately onehalt the Al, carbon not over 20%, Cb .25% in excess of 8 times the carbon, balance Fe, Mn and Si each not over .50%.

6. An electrical resistance alloy, said allo consisting of 6.5% A1, 3.25% Cr, .05% C, .65% Cb,

balance Fe.

7. An electrical resistance alloy, said alloy consisting of 6.5% A], 3.25% Cr,-.05% (1.65% Cbf Mn and Si each not over .50%, balance Fe.

8. an alloy consisting of from 5.25-7.5% Al, 24% Cr, not over 20% C, Cb 25% in excess of 8 times thecarbon. balance Fe. I

9. An electrical resistance element comprised of an alloy consisting of from 5.25-7.5% A1, 24% Or, not over 20% C, Cb 25% in excess of 8 times the carbon. Mn and Si each not over .50%, balance me.

10. An electrical resistance element comprised of an alloy consisting of 6.5% A1, 3.25% Cr, .05%

C, .65% Ch, balance Fe.

' HUGH S. COOPER.

An electrical resistance element comprised of r 

